1. Field of the Invention
The present invention relates to methods for calibrating a total-power microwave radiometer for a satellite, and more particularly to a calibration method where the calibration is carried out by deriving the brightness temperature of the cold calibration source of the total-power microwave radiometer for a satellite from equations.
2. Description of Prior Art
A microwave radiometer is a measuring apparatus which measures the brightness temperature of an object by measuring the intensity of the radio wave radiated in accordance with Plank""s radiation law from the object having a temperature, by receiving the radio wave whose intensity is approximately proportional to the temperature. The radio wave radiated from an object in accordance with Plank""s radiation law has very weak intensity, and thus is regarded as a noise in a communication and such. A microwave radiometer is an apparatus for measuring the temperature (brightness temperature) of an object under observation from remote position, by measuring such weak intensity of the radio wave accurately. As a result, a microwave radiometer is usually equipped with a cold calibration source and a hot calibration source to be used as a standard for the intensity of the radio wave measured. Among microwave radiometers, so-called xe2x80x9ctotal-power microwave radiometerxe2x80x9d is a microwave radiometer where calibrated data is inputted by using waveguides, cables or feed horns for capturing the radio wave radiated from the object under observation into the receiver, instead of inputting the calibrated data directly to the receiver.
Many of the total-power microwave radiometers mounted on a satellite utilizes the radio wave radiated from the deep space having a temperature of 2.7K as their cold calibration source. For such total-power microwave radiometers for a satellite utilizing the radio wave radiated from the deep space as their cold calibration source, the calibration of the microwave radiometer has been carried out by employing the temperature of the deep space, 2.7K, as the temperature of the cold calibration source.
In the total-power microwave radiometer which utilizes the radio wave radiated from the deep space as its cold calibration source, the cold calibration source is used in combination with a reflector, which focuses the radio wave radiated from the deep space into the feed horn. A reflector, which is smaller than the one for focusing the radio wave radiated from the object under observation into the feed horn, is used for the cold calibration source, taking both the limitation for the size and weight of the apparatus to be mounted on the satellite and the coverage of the reflector for observation of the object into consideration. As a result, the reflector for the cold calibration source has larger antenna beam width, and thus its coverage includes not only the deep space but also the body of the microwave radiometer itself and the body of the satellite, on which the microwave radiometer is mounted. In such case, since the measurement data at the cold calibration source contains the radio waves other than those radiated from the deep space, the temperature of the cold calibration source must become different value from the temperature of the deep space, 2.7K. Therefore, if one employs 2.7K as the temperature of the cold calibration source, as has been done for conventional radiometers, he or she encounters the problem that the accuracy of the measurement for the object under observation is deteriorated because of the incorporation of the error into the temperature itself to be used as a standard.
In the prior arts for solving this problem of deterioration in accuracy, there is an approach where the measurement for the deep space is carried out by changing the attitude of the satellite so that the reflector, which is normally directed to the object under observation such as the earth, is directed to the deep space, and then the data measured thereby is used for calibrating the measurement data when the cold calibration source is measured. However, in order to do this operation, one has to abort primary observation for the object under observation because changing the attitude of the satellite is necessary for this approach. In addition, since the satellite has to be forced to invert its attitude by drastic control of the attitude, the approach is very dangerous to the operation of the satellite, and is detrimental to the lifetime of the satellite. As a result, such calibration of the measurement data by changing the attitude of the satellite is rarely carried out, such as a few times at most in a mission period of several years. In addition, some satellites do not have enough capability which enables one to carry out the approach. Furthermore, even if the approach is carried out, the temperature of the cold calibration source is calibrated only at the time when the approach is carried out, and is left uncalibrated throughout the rest of the whole mission period.
There is another approach for solving the problem of deterioration in accuracy in the prior art where the temperature of the cold calibration source is calibrated by the feedback from actual measurement of the temperature of the object under observation, such as a certain area of the ocean, by synchronizing the measurement with the measurement by the microwave radiometer (sea truth). However, since the area from which the data is retrieved at one measurement by the microwave radiometer is very large, from a few kilometers square to 100 kilometers square, it is rare that the whole area has a homogeneous temperature, and it is almost impossible to understand accurately how the radio wave radiated from the area is attenuated by the atmosphere until the radio wave reaches to the microwave radiometer on the orbit so that the calibration of the temperature of the cold calibration source by the feedback can be carried out. In addition, similarly to the aforementioned approach where the attitude of the satellite is changed, the feedback calibration by this approach can be rarely carried out. Thus, even with this approach, it is impossible to determine the temperature of the cold calibration source accurately.
Accordingly, it is an object of the present invention to solve the aforementioned problems in the prior calibration method using a cold calibration source, and the present invention provides a calibration method which makes it possible to determine the temperature of the cold calibration source and to calibrate radiometer frequently.
This object is achieved by the present invention having the following features. In one embodiment, the present invention is a method for calibrating a total-power microwave radiometer having a cold calibration source and a hot calibration source and measuring brightness temperature, the method comprises the steps of measuring the brightness temperature of the hot calibration source having a temperature of Thot1 under conditions where the ambient temperature is constant; measuring the brightness temperature of the hot calibration source having a temperature of Thot2 under conditions where the ambient temperature is constant, where Thot1 and Thot2 are different temperatures; and measuring the brightness temperature of the cold calibration source where the ambient temperature is constant; repeating a predetermined number of times at least one of the steps of measuring the brightness temperature of the hot calibration source having a temperature of Thot1, measuring the brightness temperature of the hot calibration source having a temperature of Thot2, and measuring the brightness temperature of the cold calibration source; calculating the standard deviation xcex94Thot1 from the measured brightness temperatures of said hot calibration source having a temperature of Thot1; calculating the standard deviation xcex94Thot2 from the measured brightness temperatures of said hot calibration source having a temperature of Thot2; calculating the standard deviation xcex94Tcold from the measured brightness temperatures of said cold calibration source; and calibrating the total-power microwave radiometer by defining the brightness temperature of the cold calibration source according to the following equation:
Tcold=[(xcex94Thot1xe2x88x92xcex94Tcold)Thot2xe2x88x92(xcex94Thot2xe2x88x92xcex94Tcold)Thot1]/(xcex94Thot1xe2x88x92xcex94Thot2). 
In another embodiment, the present invention is a method for calibrating a total-power microwave radiometer according to the above-described embodiment, wherein the ambient temperature is constant and the steps of measuring the brightness temperature of the hot calibration source having a temperature of Thot1, measuring the brightness temperature of the hot calibration source having a temperature of Thot2, and measuring the brightness temperature of the cold calibration source are carried out while the radiometer is under sunshine or under shade.
In general, the total-power microwave radiometer is used for a satellite, and the cold calibration source utilizes radio wave radiated from the deep space. In addition, the brightness temperature of the hot calibration source or the cold calibration source is usually measured by a receiver of the radiometer. Furthermore, the temperature of the receiver is usually the same as the temperature of the ambient surrounding the receiver.
Accordingly, yet another embodiment of the present invention is a method for calibrating a total-power microwave radiometer for a satellite having a cold calibration source which utilizes radio wave radiated from the deep space and a hot calibration source and measuring brightness temperature by a receiver, the method comprises the steps of measuring the brightness temperature of the hot calibration source having a temperature of Thot1 with the receiver under conditions where the temperature of the receiver is constant; measuring the brightness temperature of the hot calibration source having a temperature of Thot2 with the receiver under conditions where the temperature of the receiver is constant, where Thot1 and Thot2 are different temperatures; and measuring the brightness temperature of the cold calibration source with the receiver under conditions where the temperature of the receiver is constant; and said method further comprises the steps of repeating a predetermined number of times at least one of the steps of measuring the brightness temperature of the hot calibration source having a temperature of Thot1, measuring the brightness temperature of the hot calibration source having a temperature of Thot2, and measuring the brightness temperature of the cold calibration source; calculating the standard deviation xcex94Thot1 from the measured brightness temperatures of said hot calibration source having a temperature of Thot1; calculating the standard deviation xcex94Thot2 from the measured brightness temperatures of said hot calibration source having a temperature of Thot2; calculating the standard deviation xcex94Tcold from the measured brightness temperatures of said cold calibration source; and calibrating the total-power microwave radiometer for a satellite by defining the brightness temperature of the cold calibration source according to the following equation:
Tcold[(Thot1xe2x88x92xcex94Tcold)Thot2xe2x88x92(xcex94Thot2xe2x88x92xcex94Tcold)Thot1]/(xcex94Thot1xe2x88x92xcex94Thot2). 
In another embodiment, the present invention is a method for calibrating a total-power microwave radiometer for a satellite according to the above-described embodiment, wherein the steps of measuring the brightness temperature of the hot calibration source having a temperature of Thot1 with the receiver, measuring the brightness temperature of the hot calibration source having a temperature of Thot2 with the receiver, and measuring the brightness temperature of the cold calibration source with the receiver are carried out while the satellite is under sunshine or under shade.